Currently, High Temperature Polymer Electrolyte Membrane Fuel Cells (HT-PEMFC) uses phosphoric acid (PA) doped poly-benzimidazole (PBI) as proton conducting membrane. These PBI based membrane electrode assembly (MEA) works even at a temperature higher than 150° C. but performance degradation of these MEAs during long term operation is a major concern and many efforts are being carried out to solve this issue. Due to this reason more research work is being focused on the electrocatalyst. The electrode as well as PA leaching from the PBI membrane during fuel cell operation, is a major concern in overall degradation of fuel cell performance.
PA is the major proton conducting source in the PBI based membranes and thus leaching of PA affects the overall performance of the MEA's. Many composite membranes have been introduced in order to improve the proton conductivity but the leaching of PA during fuel cell operation is still a pertaining issue. The formation of water vapor during fuel cell reaction on the electrode can be easily absorbed by the PA in the membrane which leads to the leaching of PA from the membrane.
Kim et. al describes the efficient formation of triple phase boundary by the incorporation of an ionomer in the catalyst layer in a modified manner. The cathode and anode are prepared by casting slurry including a catalyst and an ionomer on a gas diffusion layer, and drying the resulting layer to form a catalyst layer. The ionomer was dissolved in NMP and the Pt/C catalyst was mixed separately in NMP. After that, the two solutions were mixed well and added to a second solvent (Hexane or water) for phase separation and the ionomer film is chemically adsorbed onto the catalyst surface. This will leads to the effective covering of Pt/C by ionomer, rather than the normal method. This gives an enhanced fuel cell performance compared to electrodes made by the conventional method. (US 2006/0105226 A1, May 18, 2006)
The method comprises mixing the conductive catalyst material, the proton conductive material, and a first solvent and casting the obtained mixture onto a supporting layer. The mixture is dried to form a conductive catalyst containing film and the conductive catalyst containing film is separated from the supporting layer and pulverized. According to this invention, the ionomer percentage in compared to the conductive catalyst material is in the range of 1-50%, The ratio above or below this range would leads to a low fuel cell performance. The invention also mention about the temperature range for drying the catalyst layer after coating. The suitable temperature is 60-150° C., below 60° C. the coating would not dried well and above 150° C. the carbon support will get oxidize. (U.S. Pat. No. 8,039,414 B2, Oct. 18, 2011)
Liu et. al studied the membrane electrode assemblies in a fuel cell. They produced electrode with a good performance. In their electrode, the binder may comprise at least one triazole modified polymer which is configured to ensure that the catalyst contacts the surface of the electrolyte membrane. Here, the triazole group acts as the proton conduction path and this is effective above the boiling point of water. (U.S. Pat. No. 7,947,410 B2, May 24, 2011).
Li et. al studied the water uptake of PBI and acid doped PBI membranes. It tells that at a low acid doping percentage, the water uptake by membrane was less as the active sites of the imidazole ring was occupied with doped acid molecules. Whereas at higher acid doping level the percentage of water uptake is higher than that of nation membrane and is due to the hygroscopic nature of the acid doped with the membrane. This work also tells about the doping time required for PBI membrane and about 50 hrs is needed for doping the PBI membrane at room temperature. This work also mentioning that at higher acid doping level, the excess acid would contributing fir conductivity and also it suffer from the leaching out when sufficient liquid was present on the membrane. (Solid State Ionics 168 (2004) 177-185)
He et. al studied the conductivity of phosphoric acid doped PBI membrane with temperature, acid doping level and relative humidity. This work is also deals with the PBI composite membranes such as PBI with inorganic proton conducting materials like zirconium phosphate, phosphotungstic acid and silicotungstic acid. The conductivity of these composite membranes also studied with various parameter and obtain higher conductivity for PBI composite containing zirconium phosphate at 200° C. and 5% RH. (Journal of Membrane Science 226 (2001) 169-184)
Seland et. al studied the optimum anode and cathode composition by varying the Pt content in Pt/C and also the catalyst loading. He found that a high platinum content and a thin catalyst layer on both anode and cathode, gave the overall best performance. This was attributed to the different catalyst surface areas, the location of the catalyst in relation to the electrolyte membrane and particularly the amount of PBI dispersed in the catalyst layer. (Journal of Power Sources 160 (2006) 27-36)
Hence, a practical solution to surmount this issue to achieve successful penetration of PA-PEMFCs for commercial applications is necessary.